Recently, materials having a high light transmittance are required as optical materials. Particularly, as to a material for lenses, those having a light transmittance of 90% or more over the whole wavelength region of 400 nm to 700 nm when formed into injection-molded articles of 3 mm in thickness are considered to be desirable. That is, when the material is inferior in transmittance to a part of visible rays, a lens produced from it takes a color. Further, when the lens is used near to a strong light source, the energy of that part of visible rays of the wavelength is absorbed in the lens, converted to heat in the lens, and as a result the temperature of the lens becomes high. Therefore, there is a danger of the lens melting even if it is produced with materials which are to some degree high in heat resistance.
Hitherto known resins used as optical materials include polymethyl methacrylate (PMMA) and polycarbonate (PC). Among these, PMMA is excellent in transparency, and when formed into injection-molded articles of 3 mm in thickness, its light transmittance reaches 90% and 91% at wavelengths of 430 nm and 700 nm, respectively. PMMA, however, has a problem in terms of heat resistance and humidity resistance. On the other hand, PC is superior to PMMA in heat resistance and humidity resistance, but when formed into the same injection-molded articles as above, its light transmittance is at most about 86% at a wavelength of 430 nm. Further, when it is molded into lenses, there is a problem of birefringence being large.
Recently, the hydrogenated product of a thermoplastic norbornene polymer has attracted attention as an optical material excellent in heat resistance, humidity resistance and low birefringence. However, the hydrogenated product of a thermoplastic norbornene polymer produced by the conventional methods was only such that when formed into injection-molded articles of 3 mm in thickness, its light transmittance is less than 90% at a wavelength of 430 nm, although its light transmittance is 90% or more at a wavelength of 700 nm. Further, when an optical element for information recording media is produced by vapor-depositing a metallic film onto a substrate made of the hydrogenated product of a thermoplastic saturated norbornene poller, there is a case where adhesion of the metallic film to the substrate is not always sufficient as can be seen in the generation of blister owing to the partial peeling-off of the metallic film under high-temperature and high-humidity conditions. Improvement of such the drawback has therefore been demanded.
As described later, the present inventors have found that the transparency and adhesion to metallic film of the hydrogenated product of a thermoplastic norbornene polymer can be improved by reducing the content of a transition metal atom present as polymerization catalyst residues in the thermoplastic norbornene polymer. The methods so far used to reduce this content include the following: A method of washing the polymer solution with a poor solvent; a method of adding a small amount of the poor solvent to the polymer itself, dissolving the polymerization catalyst in the poor solvent and separating the catalyst solution; and a method of treating the polymerization solution with an adsorbent (e.g. activated alumina, zeolite, etc.) in the presence of a compound having a hydroxy group (Japanese Patent Application Kokai No. 3-66725). If the polymerization catalyst is removed by these methods, however, the content of the transition metal atom present as polymerization catalyst residues in the polymer treated as above is about 2 ppm or more. At present, therefore, such the hydrogenated product that the content of the transition metal atom is less than about 2 ppm also has not been obtained from this thermoplastic norbornene polymer.
A method of using a heterogeneous catalyst in the hydrogenation of the thermoplastic norbornene polymer is disclosed in Japanese Patent Application Kokai No. 1-311120, No. 3-66725, etc. in which carbon, silica, alumina, titania, etc. are used as a carrier. These so far used carriers, however, are too short of pore volume and specific surface area to give an adsorbing ability to them, so that examples are not known where a heterogeneous catalyst comprising a catalytic metal supported on a carrier having a large pore volume and specific surface area was used.